Noise from engine after ignition off

TheBlacklist2013

I had engine check light on a few days ago and diagnosis confirms faulty Mass air flow sensor.

I am getting it replaced in next couple of days, however since today, I have started noticing this sound once the car is switched off... is it something with MAF or another issue

Does anyone know what could it be?

Here is a video link

https://quickshare.samsungcloud.com/vV53SK7JahK3


No other issues while driving though.

Goudy

It's sounds like it's running on or what is sometimes known as dieseling.
Unburnt fuel is igniting under compression after you have turned the ignition off.

It's sometimes called dieseling as a diesel engine is designed to run this way, air is compressed and gets mad hot, fuel is then added to the hot air and that causes the ignition. They don't use a spark to ignite the fuel, just the heat generated by compressing all the air in the cylinders.

There are a few reasons they do this but modern petrol cars don't usually suffer as much due to modern fuel injection systems replacing carburettors as they are now less able to draw in fuel once the ignition is turned off.

Though it can still happen if say the engine is running really lean.
The lean running engine causes excess heat in the combustion chambers that helps ignite any unburnt fuel in there without a spark and you get the stumbling run until it's used up that small about of unburnt fuel when you switch off.

It's likely due to your MAF problem, there are other reasons like valve damage, but with the MAF playing up that's the first point of call.

The engine ECU can no longer reliably measure the different amounts of air entering the engine when the throttle is open or closed, so it uses an emergency pre-set value to help you struggle home.
That is likely to cause it to run rich at low rpm and lean at higher rpm.
Rich tends to cause cooler combustion, lean far hotter combustion, if it's hot enough it will ignite any fuel hanging around.

Of course diesels run like this anyway,  so any excess heat in the cylinders and any unburnt fuel in there will cause the same issue.

facade

Sounds like a diesel, and it is running on (I'd have liked to hear it until it stopped.

I'd check that the oil level.

Could be too full and excess gets into the cylinder, or could be low because the turbo is leaking oil.

Other than that, it is a Renault engine. I'd check all the earths and wiring very carefully for corrosion

Goudy

I think it is a diesel, by the looks of it a 1.6 DCi.

Check the oil level and then see what it runs like after the new MAF is fitted.

TheBlacklist2013

I have noticed that this sound doesnt always come. Like I started the car earlier today for few minutes but there was no sound when I switched it off. 

The MAF will be replaced on Monday morning so is it okay if I drive the locally (on need basis) doing not more than 25mph, and is there a risk that this would cause further damage? I have car booked for service tomorrow. 

Thanks 

TheBlacklist2013

BTW, the Oil level, coolant are fine. 

TheBlacklist2013

so went to the Garage today. The main guy who was on leave last week was in, and said he'd inspect the pipes before replacing the air MAF sensor. 

He said there is a leak in Boost pipe that is causing the air pressure to drop internally resulting in engine check light. He said usually the MAF sensors should last for car's life and they are usually not replaced. 

What to do? 

Goudy

That would have the same effect on the fuel/air ratio.

The MAF measures the air entering the engine, which the ECU then fuels for.
Any extra air being sucked in after the MAF (usually before the turbo) or any air leaking away before it gets to combustion (usually after the turbo) will alter that fuel/air ratio at it can no longer detect the correct levels of air.

It's not uncommon for boost pipes (after the turbo) to split but you usually hear a whoosh or rasp when accelerating as the boosted air escapes, plus the power drops off as the engine is no longer getting that boosted air, though it has measured for it.

You'll end up with a rich mix and a could have a bit of unburnt fuel hanging around after you have switched off, hence the run on.

If it's sucking in air between the MAF and turbo, you'll have excess air meaning a lean and hot combustion which again can cause the run on after turning the ignition off.

If either is happening, what the ECU sees as a signal from the MAF won't be what the ECU expects to see, so that could throw up a MAF related diagnostic code.

You were told it was a MAF fault code, but that could be one of many. Without the code it's hard to tell what might be wrong.
Generally sensor faults are under/over expected values or an open circuit.

You can probably guess the next bit.
An open circuit is a duff sensor or wiring problem, there is nothing coming back from it.

And under or over value reading could be a duff sensor, MAFs can get contaminated with dirt and oil but it could also be a working sensor but it's just picking up an issue that is causing that under/over value reading, like a leak.

If they think there's a boost leak, that needs sorting first.
Then the trouble code clearing and see if it runs ok without further MAF related trouble codes.

If the MAF code keeps coming back, the wiring is next to look at, then the MAF itself.

TheBlacklist2013

Thanks for detailed reply and guidance. 

This is the pipe that has leak. When I rev it up, you can feel air escaping from this. Can any1 confirm OE part number for this pipe? I am getting different info with regards to this. Some call it boost pipe, some intercooler hose, some Charge air pipe...

Its 2015 Nissan Qashqai 1.6 Diesel

Goudy

All the names are right.
Some call it an intercooler pipe, others a boost pipe or charged air pipe.

There will be two lengths of hose to the intercooler, one from the turbo to the cooler and one from the cooler to the manifold.

I seem to think there are various pieces that make up that particular hose as the longer sections tend to be hard plastic and the joins at the end rubber hose.
So you'd need to find which part is split, that will probably mean taking it apart.

I think there are a couple of companies that make a better replacement for some or all the various pieces.
Normal rubber hose tends to degrade over time particularly when it gets a bit oily from the crankcase blowby.
Silicone replacements tend to last longer as they resist oil better and flex a little more.

TheBlacklist2013

Thanks Goudy for continuous guidance. 

The leak is just under the highlighted side of the pipe, as it starts bending down towards the engine. Nissan quoted £336 for this to the Garage yesterday